A raindrop sensor disclosed in JP-A-2001-153969 captures an image of a vehicle windshield and performs a frequency analysis of the image. When a raindrop is on the windshield, the raindrop appears as a high-frequency component in the image as a result of the frequency analysis. The raindrop sensor detects the presence and amount of the raindrop based on the amplitude of the high-frequency component. However, the raindrop sensor may incorrectly detect a foreign matter (e.g., water stain) on the windshield as the raindrop, because the foreign matter may appear as the high-frequency component. Further, since the frequency analysis relies on an outline of the raindrop, the raindrop may not appear as the high-frequency component in the case where the outline of the raindrop is blurred. Therefore, if a large portion of the windshield is covered with water, for example, by a splash from a passing vehicle, the raindrop sensor may not detect the splash.
An image processing system disclosed in US 2006/0243894A1 corresponding to JP-A-2005-195566 includes a lamp for emitting light to a windshield and an image sensor for capturing an image of the light reflected from the windshield. A raindrop on the windshield is detected by analyzing the image. However, the image processing system is based on the assumption that the windshield is water-repellent and the raindrop on the windshield is round like a ball. Therefore, in the case where the windshield is not water-repellent or the windshield is covered with water, the image processing system may not accurately detect the raindrop.